The invention relates to a d.c. supply arrangement for an S-bus connected to an ISDN network termination, including a first d.c. supply unit supplying a d.c. voltage intended for the S-bus, this unit being fed from a power source which is independent of the ISDN network, and the arrangement including a second d.c. supply unit supplying also a d.c. voltage intended for the S-bus, this second unit being fed from the ISDN subscriber line, the output of the first d.c. supply unit and the output of the second d.c. supply unit being coupled with opposite polarity to the bus via a coupling circuit which connects one of the two d.c. supply units to the S-bus in dependence on the value of the d.c. output voltage of the first d.c. supply unit.
Such a d.c. supply arrangement is known from the "Conference Proceedings Intelec 85", Munich, F.R. of Germany, Oct. 14-17, 1985, pp. 505-512, more specifically FIG. 6 and the associated text.
In an lSDN network, the subscriber line ends in a network termination (NT) at the subscriber's location. At the subscriber's location the NT is connected to one or a plurality of terminals through the S-bus. These terminals usually consist of one or a plurality of telephone sets and, for example, a personal computer (PC).
The terminals are fed through the S-bus by the d.c. supply arrangement connected to the S-bus. Thus, a plurality of terminals, independent of their position on the S-bus, can be fed by a single d.c. supply arrangement.
In this respect it is attractive to provide the d.c. supply arrangement with two d.c. supply units which are led from mutually independent power sources. If the first d.c. supply unit shows a failure, the second d.c. supply unit can take over the supply to the S-bus. By coupling the second d.c. supply unit to the S-bus with a polarity opposite to that of the first d.c. supply unit it is achieved that the second unit supplies power only to those terminals which have been made suitable for a supply voltage with opposite polarity. This polarity reversal is recommended by the CCITT (Recommendation I. 430, Paragraph 9).
In the aforementioned "Conference Proceedings Intelec '85" the first d.c. supply unit, to be called the a.c./d.c. converter hereinafter, takes its power from the mains. The second d.c. supply unit, to be called the d.c./d.c. converter hereinafter, takes its power from the subscriber line.
The two converters are connected to the S-bus through a coupling circuit. In the "Conference Proceedings Intelec '85" the coupling circuit is formed by a mechanical change-over switch. The positive output conductor of one converter is connected to the negative output conductor of the other converter and also to a conductor of the S-bus. The other two output conductors are connected to different selector contacts of the change-over switch. The main contact of this change-over switch is connected to the other conductor of the S-bus. Thus, the two converters are coupled with the opposite polarity to the S-bus and the aforementioned Recommendation of the CCITT is satisfied.
In addition to the widely known disadvantages of mechanical switches, the mechanical change-over switch forming the coupling circuit is further disadvantageous in that, if the two converters are positioned at a large relative distance, a long feeding line between the converter and the change-over switch will be required in addition to the S-bus. The fact that the converters have to be positioned at a large relative distance occurs, for example, if there is no mains voltage in the neighbourhood of the network termination
The invention has for its object to provide a d.c. supply arrangement in which the two d.c. supply units are directly electrically coupled to the S-bus.
To accomplish this, the d.c. supply arrangement according to the invention is characterized in that the two d.c. supply units each comprise a series transistor associated to the coupling circuit, the main current path of this transistor being serially connected to an output conductor of the associated d.c. supply unit, in that the coupling circuit further includes a monitoring circuit with two input conductors which are connected to the output of the first d.c. supply unit and with a control output which is connected to the control electrode of the series transistor associated with this d.c. supply unit to allow the latter to block in case the d.c. output voltage of the d c. supply unit falls short of a preset threshold, and :n that the coupling circuit includes a threshold circuit having at least a first and a second input conductor which are connected to the main electrodes of the series transistor associated with second d.c. supply unit and having a control) output which is connected to the control electrode of this series transistor for bringing the latter into the conductive state in case the voltage over the main electrodes of the series transistor falls short of a preset threshold.
By causing the series transistor associated with one d.c. supply unit to assume the blocking state when the series transistor associated with the other d.c. supply unit is in the conductive state, it is achieved that the d.c. supply units do not mutually form a load.
When the first d.c. supply unit (the a.c./d.c. converter) functions in the normal way, the monitoring circuit detects a sufficiently high output voltage of the a.c./d.c. converter. The monitoring circuit renders the associated series transistor conductive causing the a.c./d.c. converter to feed the S-bus.
Since the a.c./d.c converter and the d.c./d.c. converter are coupled with opposite polarity to the S-bus there will be a difference in voltages between the main electrodes of the other series transistor which difference is substantially equal to the sum of the output voltages of the a.c./d.c. converter and the d.c./d.c. converter. The threshold circuit connected to these main electrodes detects this relatively high voltage and causes the series transistor associated with the d.c./d.c. converter to block, so that the d.c./d.c. converter does not form a load for the a.c./d.c. converter.
Once the output voltage of the a.c./d.c. converter falls short of a preset threshold, the monitoring circuit causes the series transistor associated with the a.c./d.c. converter to block. Consequently, the current path between the a.c./d.c. converter and the S-bus is blocked. Owing to this, the a.c./d.c. converter no longer feeds the S-bus.
Consequently, the voltage over the electrodes of the series transistor associated with the d.c./d.c. converter drops to the output voltage of the d.c./d.c. converter. The threshold circuit detects this voltage drop and in response to this renders the associated series transistor conductive. A current path develops between the d.c./d c. converter and the S-bus so that the d.c./d.c. converter feeds the S-bus
Because the series transistor associated with the a.c./d.c. converter is blocking, the a.c./d.c. converter does not form a load for the d.c./d.c. converter feeding the S-bus.
A preferred embodiment of the d.c. supply arrangement according to the invention is characterized in that the monitoring circuit comprises a voltage reducer, a voltage reference circuit and a comparator, in which the input conductors of the voltage reducer form the input conductors of the monitoring circuit, in which the output of the voltage reducer is connected to one of the inputs of the comparator to whose other input is connected the voltage reference circuit, and in that the output of the comparator forms the control output of the monitoring circuit.
As is widely known, a voltage reducer can be constituted by, for example, a resistance voltage divider or by a d.c. amplifier having a gain factor smaller than one. The voltage derived from the output voltage of the a.c./d c. converter by the voltage reducer is compared by the comparator to a reference voltage coming from the voltage reference circuit. If the voltage derived from the output voltage exceeds the reference voltage, the comparator will command the control electrode of the associated series transistor such that the latter becomes conductive. In this case the S-bus is fed by the a.c./d.c. converter.
Once the derived voltage drops below the reference . voltage, the comparator commands the control electrode of the series transistor such that the latter blocks. The current path between the a.c./d.c. converter and the S-bus is blocked. The S-bus is then no longer fed by the a.c./d.c. converter.
A preferred embodiment of the d.c. supply arrangement according to the invention is characterized in that the threshold circuit comprises a control transistor, a base series resistor and a voltage divider, the voltage divider comprising two input conductors forming the input conductors of the threshold circuit, the output of the voltage divider being connected to the control element of the control transistor, one main electrode of this control transistor being connected to the input conductor of the threshold circuit which is directly connected to an output conductor of the second d c supply unit, the other main electrode of this control transistor forming the control output of the threshold circuit, and in that the base series resistor is arranged between this control output and the other output conductor of the second d.c. supply unit.
As is widely known, a voltage divider can be formed by a resistance voltage divider.
Under normal operating conditions the a.c./d.c. converter feeds the S-bus. As described hereinbefore, there is then a voltage difference over the main electrodes of the series transistor associated to the d.c./d.c. converter. This voltage difference is substantially equal to the sum of the output voltages of the a.c./d.c. converter and the d.c./d.c. converter.
The voltage derived from this relatively high difference in voltages by the voltage divider is applied to the control electrode of the control transistor. The dividing ratio of the resistors in the voltage divider is chosen such that in this case the derived voltage moves the control transistor into the conductive state. Consequently, a relatively large current starts to flow through the main current path of the control transistor and through the base series resistor. Because the control electrode o: the series transistor is connected to the junction of the control transistor and the base series transistor, the voltage difference over the base series resistor due to the relatively high current through the base series resistor causes such a voltage on the control electrode of the series transistor that the latter is blocking. In this case the current path between the d.c./d.c. converter and the S-bus is blocked, so that the d.c./d.c. converter does not form a load for the a.c./d.c. converter.
When the voltage difference over the main electrodes of the series transistor associated with the d.c./d.c. converter drops owing to the fact that the series transistor associated with the a.c./d.c. converter is blocking, which was described hereinbefore, the voltage derived from this difference in voltages by the threshold circuit will drop likewise. Now such a voltage develops on the control electrode of the control transistor that in response thereto the control transistor begins to block. Therefore, no current will flow any longer via the main current path of the control transistor through the base series resistor. The voltage over the base series resistor drops and since the control electrode of the series transistor is connected to the junction of the control transistor and the base series resistor, a current will start flowing through the control electrode, and the series transistor will become conductive. The d.c./d.c. converter will now start supplying power to the S-bus.
A further embodiment of the invention is characterized in that the threshold circuit comprises three input conductors, in that one main electrode of the series transistor associated with the second d.c. supply unit via an emitter series resistor is connected to an output conductor of this d.c. supply unit and in that the junction between the second d.c. supply unit and the emitter series resistor is connected to the third input conductor of the threshold circuit, and in that the threshold circuit comprises a control transistor, a base series resistor and a voltage divider, the latter comprising two input conductors forming the first and the second input conductor of the threshold circuit, the output of the voltage divider being connected to the control electrode of the control transistor, one main electrode of this control transistor being connected to the third input conductor of the threshold circuit, the other main electrode of this control transistor forming the control output of the threshold circuit, and in that the base series resistor is arranged between this control output and the other output conductor of the second d.c. supply unit.
If there is a current path between the d.c./d.c. converter and the S-bus and thus the d.c./d.c. converter feeds the Sbus, the power supplied to the S-bus flows through the emitter series resistor. This power causes a voltage difference over the emitter series resistor. As long as the load resistance connected to the S-bus has a sufficiently large value, this current remains sufficiently small, and the effect of the emitter series resistor is negligible.
When the load resistance assumes too small a value, the current supplied to the S-bus becomes so large as to have the consequent voltage difference over the emitter series resistor render the control transistor conductive. This causes a current to flow via the control transistor through the base series resistor across which this current causes a voltage difference. Consequently, the series transistor becomes less conductive. Therefore, no rise of the current through the series transistor can be realized if the load resistance connected to the S-bus assumes an even smaller value. Thus, the emitter series resistor limits the current flowing through the series transistor to a constant value if the load resistance assumes too small a value.
A further embodiment according to the invention is characterized in that the monitoring circuit further includes a switching transistor whose one main electrode is connected to the input conductor of the voltage reducer which is also connected to the positive output of the first d.c. supply unit and whose other main electrode is connected to the control output of the voltage reducer and whose control electrode is connected to the comparator output.
Once the monitoring circuit detects a sufficiently high output voltage of the a.c./d.c. converter, this monitoring circuit renders the associated series transistor conductive.
Because the control electrode of the switching transistor is connected to the comparator output, the switching transistor will also be rendered conductive.
The control output of the voltage reducer is connected via this conductive switching transistor to the input converter of the voltage reducer to which also the main current path of this switching transistor is connected. The current now flowing through the switching transistor changes the output voltage of the voltage reducer such that the difference between the voltages at the inputs of the comparator is enhanced. Because the series transistor is conductive, the a.c./d.c. converter supplies current to the S-bus. This current may cause a voltage drop over an optional internal resistance of the a.c./d.c. converter. Thereby, the output voltage of the a.c./d.c. converter would drop. The attendant voltage drop at the control output of the voltage reducer is now compensated by the enhancement as described hereinbefore. This achieves that if the a.c./d.c. converter supplies current to the S-bus, the voltage loss over the internal resistance of the a.c./d.c. converter caused by this current does not result in the monitoring circuit causing the series transistor to block again as a consequence of this voltage drop. In this way a hysteresis effect is obtained.
Conversely, when the output voltage of the a.c./d.c. converter falls short of a preset value, the comparator will cause the series transistor and the switching transistor to block. An optional internal resistance will cause the output voltage to rise, which rise is compensated by the :act that the switching transistor begins to block. Thus, also in the opposite direction the monitoring circuit shows a hysteresis effect.
This hysteresis effect is advantageous in that the functioning of the monitoring circuit becomes independent of an internal resistance that is not negligibly small and may be present in the a.c./d.c. converter. In addition, slight fluctuations in the mains voltage around the switching point of the output voltage (which is the value of the output voltage of the a.c./d.c. converter at which the monitoring circuit moves the series transistor from the non-conductive to the conductive state and vice versa) will not result in a constant undesired switching of the series transistor .